Computer memory devices pose unique challenges in yield improvement. Because memory devices often contain redundant rows and columns, they are capable of being repaired to a limited extent. A “repair” involves deactivating malfunctioning memory cells and replacing them with available redundant cells that are functional. Typically, repairing involves deactivating one or more bad rows and/or columns of data storage locations on the chip and transferring the functionality of the bad rows and/or columns to the redundant rows and/or columns. This can be done, for example, by applying predetermined voltage to specific locations on the chip causing fuses formed into the circuit to disconnect, thereby modifying the circuit.
FIG. 1 shows an exemplary memory device 10 having a main memory region 12 divided into sectors 14. Memory device 10 has a region of redundant rows 18 and a region of redundant columns 20. Redundant rows 18 and columns 20 can be used in place of faulty rows and columns in main memory region 12. In addition, one or more sectors 16 of sectors 14 can be switched off, thereby causing them to be bypassed or ignored in operation. For example, if errors in sector 16 cannot be repaired by using redundant rows or columns, then it can be switched off, and memory device 10 can be converted into a partially-good memory (PGM) device that has less memory capacity than the full designed capacity.
Previously, there has been no reliable approach to predicting the impact of process modification after the repair, or to what extent defective devices can be sold as PGM after repair. The term, “process,” refers to the manufacturing process used in producing the memory device. FIG. 2 shows a bar graph presenting exemplary failure rates for a plurality of memory chips. Prior to repair, there are far more single bit failures 32 than any other failure mode. Based on this data, it would appear that the production process should be modified to correct for single bit failures in order to have the greatest yield improvement. However, after repair, it is clear that column patterns 34 are the greater concern since single bit failures can more often be successfully repaired. Therefore, considering the data after the repair, it would make more sense to modify the production process to correct for column patterns. It should be recognized that the data presented in FIG. 2 is for illustration purposes only, and that it represents a real-world problem in simplistic terms. Different types of memory devices have differing failure modes. A process modification for correcting one failure mode can have a negative impact in other failure modes. Unfortunately, the amount of improvement and the amount of negative impact cannot be quantified using current analysis methods because process engineers cannot see beyond the repair to predict the final effect of the process changes.
Furthermore, memory devices can still be sold as partially-good memory even if they do not operate to specifications. PGM devices may have one or more sectors of memory turned off, so they essentially provide less capacity than the maximum capacity possible provided for by the design or specification. This can further complicate the modification of the manufacturing process since more failures of a particular type may be tolerated if the device can still be sold as a PGM device. Again, without knowing the status of the device after being repaired, a sufficiently informed decision cannot be made as to process modifications.